Sheet atacker and finisher

ABSTRACT

An improved sheet stacking scheme is described which is applied to a copying machine, a printer, or the like. Even when printed sheets are sorted and stacked in different stacking positions, sheet alignment is achieved by inhibiting each sheet from being displaced. When sheets are sorted into a first stacking position and a second stacking position, the finisher control unit  200  determines the common area Rc which is shared by the first stacking position and the second stacking position. Then, a finisher control unit  200  sets the positions of the pair of holding members for holding a tail edge in order to perform the tail edge holding motion in the positions F 1  and F 2  respectively which are located within the common area Rc and symmetrical about the center position Rcc of the common area Rc.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. P2011-136047, filed Jun. 20, 2011. The contents of this application are herein incorporated by reference in their entirety.

FIELD OF INVENTION

The present invention relates to a sheet stacker and a finisher, for example, provided as a printer accessory or the like.

DESCRIPTION OF RELATED ART

Conventionally, it is known that some image forming system includes a finisher for performing a variety of post-printing processes in addition to an image forming apparatus as an electrophotographic system. Also, this image forming system is provided with a sheet stacker in the final stage of the system (for example, in a finisher). The sheet stacker serves to stack sheets of paper which are output from a paper discharge unit. The sheet stacker is configured to move a catch tray up and down for stacking printed sheets in the stacking direction to make it possible to discharge a large amount of sheets. In addition to this, this sheet stacker is configured to sort printed sheets in different stacking positions by performing shift operations in a predetermined cycle. The shift operation is performed, for example, to move the sheet transporting position or the catch tray in the sheet width direction (the direction perpendicular to the sheet transportation direction) by a predetermined amount with a predetermined timing.

For example, Japanese Patent Application Laid-Open Publication No. 2009-23739 discloses a sheet stacker capable of performing the shift operation. According to this sheet stacker, each time a printed sheet is discharged, holding members are driven to hold the upper surface of the sheet downwards against a catch tray (or the previously uppermost sheet) near the tail end of the sheet as seen from the transportation direction of the sheet. This structure makes it possible to prevent a sheet from being displaced when the sheet falls on the catch tray, and thereby sheet alignment can be achieved to a desired level.

However, while the technique described in the publication is effective to inhibit sheet misalignment when a sheet falls on the catch tray, the shift operation is not taken into consideration.

SUMMARY OF THE INVENTION

To achieve at least one of the abovementioned objects, a sheet stacker reflecting one aspect of the present invention comprises a catch tray configured to stack a sheet discharged from a discharging unit; a holding unit including a first holding member and a second holding member which are arranged adjacent to each other in the sheet width direction that is perpendicular to the discharge direction of the sheet; a lateral motion drive unit configured to move said first holding member and said second holding member in the sheet width direction respectively; a holding drive unit configured to drive said first holding member and said second holding member to hold down the tail edge area of the sheet from the upper side of the sheet toward a sheet receiving surface; and a control unit configured to control said holding drive unit to perform a tail edge holding motion in synchronization with the discharge of the sheet, and control said lateral motion drive unit. Particularly, when the sheet is stacked by sorting between a first stacking position and a second stacking position which is offset from said first stacking position in the sheet width direction, said control unit determines a common area which is shared by said first stacking position and said second stacking position, and sets the positions of said first holding member and second holding member in order that said tail edge holding motion is performed in symmetric positions which are located within said common area and symmetrical about the center position of said common area.

Preferably, said holding unit comprises three holding members functioning as said first holding member, said second holding member and a third holding member which is arranged in said sheet width direction with said first holding member and said second holding member.

When said stacking position is switched, said control unit performs a shift position holding motion by holding the side edge area of an uppermost sheet stacked in the stacking position before switching with said third holding member, in the side opposite the shift direction of said stacking position.

Preferably, said control unit assigns, of said three holding members constituting said holding unit, a pair of adjacent holding members to said first holding member and said second holding member for performing said tail edge holding motion and the remaining holding member to said third holding member for performing said shift position holding motion.

Preferably, when said stacking position is switched, said control unit changes assignment of said three holding members constituting said holding unit in correspondence with the stacking position after switching.

Furthermore, said control unit sets the positions of said three holding members in accordance with said assignment thereof as changed and the stacking position after switching.

Preferably, said control unit sets a holding pressure, with which said shift position holding motion is performed by said holding members, to be smaller than a holding pressure with which said tail edge holding motion is performed by said holding members.

Preferably, the sheet stacker of claim 1 further comprises: a pair of regulating members configured to align a sheet discharged from said discharging unit in the sheet width direction; and a regulating member drive unit configured to move said regulating members in the sheet width direction respectively.

Particularly, said control unit controls said regulating member drive unit to set the positions of said pair of regulating members in correspondence with a stacking position of the sheet.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory view for schematically showing the overall configuration of an image forming system in accordance with an embodiment of the present invention.

FIG. 2 is an explanatory view for schematically showing the configuration of a sheet stacker of the image forming system.

FIG. 3 is an explanatory view for showing the operation of regulating members of the image forming system.

FIG. 4 is an explanatory view for showing the operation of holding members of the image forming system.

FIG. 5 is a block diagram for schematically showing the control architecture of holding members of the image forming system.

FIG. 6 is a view for explaining a scheme to determine the common area and the positions of the holding members for holding a tail edge of a sheet.

FIG. 7 is a flow chart for showing the stacking operation of the image forming system.

FIG. 8 is a timing chart showing the stacking operation of the image forming system.

FIG. 9 is an explanatory view for showing the positions of a pair of holding members for holding a tail edge, and the position of a holding member for holding a shift position.

FIG. 10 is an explanatory view for showing the situation where the tail edge holding motion is performed in a position outside the common area Rc.

FIG. 11 is an explanatory view for showing the sheet stacking state in which the holding member for holding a shift position is not used.

FIG. 12 is an explanatory view for showing the positions of a pair of the holding members for holding a tail edge and the position of the holding member for holding a shift position.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is an explanatory view for schematically showing the overall configuration of the image forming system in accordance with the present embodiment. This image forming system includes an image forming apparatus 1 and a finisher 2.

The image forming apparatus 1 is, for example, a copying machine, a printer, a facsimile machine, a multi-functional peripheral, or the like which is provided with an electrophotographic image forming apparatus for forming images on sheets P on the basis of image data. The image forming apparatus 1 includes an original document reading unit 5, a photoreceptor unit 11, a charging unit 12, an image exposure unit 13, a developing unit 14, a transfer unit 15A, a separation unit 15B, a cleaning unit 16, a fixing unit 18 and an image formation control unit 100 (refer to FIG. 5).

The original document reading unit 5 is located on the top of the housing of the image forming apparatus 1, and provided with an automatic document feeder for automatically feeding original documents to this feeder when scanning images thereof. This original document reading unit 5 reads an image from an original document and outputs a predetermined image signal. The output image signal is A/D converted into image data.

The original document reading unit 5 is provided with an image reading control unit 110 (refer to FIG. 5) which processes the image signals by performing shading compensation, dithering, data compression and so on, and outputs the processed image signals to the image formation control unit 100 as final image data. Incidentally, the image formation control unit 100 may receive image data not only from the image reading control unit 110, but also from another image forming apparatus, a personal computer or the like connected to the image forming apparatus 1.

The surface of the photoreceptor unit 11 is uniformly charged with electricity by the charging unit 12. The image exposure unit 13 scans and exposes the surface of the photoreceptor unit 11 with laser beams on the basis of the image data in correspondence with the output information which is output from the image formation control unit 100. This process forms latent images on the photoreceptor unit 11 uniformly charged with electricity. The developing unit 14 performs reversal development of the latent image with toner to form a toner image on the surface of the photoreceptor unit 11.

The sheet P stored in one of paper trays 17A is fed to the transfer unit 15A. The transfer unit 15A transfers the toner image on the surface of the photoreceptor unit 11 to the sheet P, and then the separation unit 15B separates the sheet P from the photoreceptor unit 11 with the toner images transferred to the sheet P. The cleaning unit 16 removes toner remaining on the surface of the photoreceptor unit 11 after the transfer unit 15A transfers the toner images to the sheet

The intermediate conveyance unit 17B transports the separated sheet P to the fixing unit 18. The fixing unit 18 performs a fixing process by heating and pressing the sheet P. The discharging unit 17C discharges (supplies) the sheet P after the fixing process to the finisher 2.

On the other hand, when images are formed on both sides of the sheet P, a transportation route switch plate 17D is driven to switch the transportation route of the sheet P from the direction toward the discharging unit 17C to the downward direction (toward the sheet reversing conveyance unit 17E) after performing the fixing process of the sheet P by the fixing unit 18. The sheet reversing conveyance unit 17E transports the sheet P to the transfer unit 15A after reversing the front and back sides of the sheet P by a switchback operation. The transfer unit 15A transfers a toner image to the back side of the sheet P, which is then passed through the fixing unit 18 and supplied from the discharging unit 17C to the finisher 2.

The finisher 2 is located in the subsequent stage of the image forming apparatus 1, and performs post-printing processes of the sheet P discharged from the image forming apparatus 1. The finisher 2 of the present embodiment includes a post-printing unit for performing a post-printing process, for example, a staple process, i.e., the process of stapling a plurality of printed sheets P together. The finisher 2 is comprised mainly of a sheet receiving unit 20, an intermediate stacker 40, a staple unit 43, a sheet stacker 50 and a finisher control unit 200 (refer to FIG. 5).

The sheet receiving unit 20 is provided to receive and introduce the sheet P, which is output from the image forming apparatus 1, into the finisher 2. The position of this sheet receiving unit 20 is determined to be aligned with the discharging unit 17C of the image forming apparatus 1.

Next, the transportation route of the sheet P introduced from the sheet receiving unit 20 is explained. The transportation route in the downstream of the sheet receiving unit 20 branches into three routes, i.e., a first transportation route R1, a second transportation route R2 and a third transportation route R3. The sheet P transported through the sheet receiving unit 20 is supplied to one of the transportation routes R1 to R3 in accordance with the position of a switch gate (not shown in the figure). When the sheet P is discharged to an outside tray without a stapling operation, the switch gate is set to the position to lead to the first transportation route R1 or the second transportation route. On the other hand, when a stapling operation is performed, the switch gate is set to the position to lead to the third transportation route R3. Incidentally, even when the sheet P is discharged to an outside tray without a stapling operation, if a shift operation to be described below is to be performed, the switch gate is set to the position to lead to the third transportation route R3.

The transportation routes R1 to R3 are made up of a number of conveyance rollers and guide members. The first transportation route R1 is a route for transporting the sheet P from the sheet receiving unit 20 to a subordinate catch tray 60. This subordinate catch tray 60 is arranged outward in an upper location of the finisher 2. The subordinate catch tray 60 has a small capacity, and therefore is used for discharging a small number of unusual sheets, for example, sheets of cardboard.

The second transportation route R2 is a route for transporting the sheet P from the sheet receiving unit 20 to a catch tray 51 of the sheet stacker 50. The second transportation route R2 terminates in a first discharging roller 25 serving as a discharging unit. The first discharging roller 25 discharges the sheet P, which is transported along the second transportation route R2, to the catch tray 51.

The third transportation route R3 is a route for transporting the sheet P from the sheet receiving unit 20 to the intermediate stacker 40. The third transportation route R3 terminates in a second discharging roller 26 serving as a discharging unit. This second discharging roller 26 is located above the intermediate stacker 40 (specifically speaking, located in the upper side of sheets stacked on the intermediate stacker 40), and driven to discharge the sheet P, which is transported along the second transportation route R3, to the intermediate stacker 40. The sheet (or stack of sheets) on the intermediate stacker 40 is transported by a paper feed unit 42, and thereafter discharged to the catch tray 51 by the discharging roller 25 which is part of the second transportation route R2.

Incidentally, the finisher 2 is also provided with a paper supplier unit 30 in order to enable the finisher 2 to receive sheets from a different route than the sheet P is discharged from the image forming apparatus 1. This paper supplier unit 30 is comprised of a paper supply tray and a paper feeding mechanism. The sheet P stacked on the paper supply tray is taken by the paper feeding mechanism, and thereafter transported through a predetermined transportation route to the transportation route in the downstream side of the sheet receiving unit 20.

The intermediate stacker 40 is responsible for successively accumulating the sheet P discharged from the second discharging roller 26 to supply the accumulate sheets to the staple unit 43 for performing an staple operation. Also, when a shift operation to be described below is to be performed, this intermediate stacker 40 is responsible for temporarily saving the sheet P to offset the transportation position of the sheet P in the sheet width direction (the direction perpendicular to the sheet transportation direction) corresponding to the stacking position of the sheet P.

The intermediate stacker 40 is obliquely arranged to in order that the sheet P stacked on this intermediate stacker 40 is oriented upward with its leading edge being located upper than its tail end. The sheet P discharged from the second discharging roller 26 falls on the intermediate stacker 40 or the uppermost sheet stacked thereon. The fallen sheet P slides down the inclined surface of the intermediate stacker 40 or the upper surface of the uppermost sheet, and stops when the tail end of the sheet P abuts a lower guide plate (not shown in the figure).

Around the intermediate stacker 40, a sheet aligning member 41 is arranged together with the paper feed unit 42. The sheet aligning member 41 aligns the position of the side edges of the sheet P successively accumulated on the intermediate stacker 40 (the edges of the sheet P oriented in parallel with the sheet transportation direction) for performing a staple operation.

The sheet aligning member 41 is made up of, for example, a pair of movable plates which move back and forward in the sheet width direction. The sheet aligning member 41 is driven each time the second discharging roller 26 discharges the sheet P, and aligning the side edges of the discharged sheet P with the side edges of the uppermost sheet P stacked on the intermediate stacker 40 by abutting the side edges of the discharged sheet P respectively.

Also, when a shift operation is to be performed, this sheet aligning member 41 is driven each time the second discharging roller 26 discharges the sheet 2, and sets the transportation position of the sheet P in the sheet width direction. Specifically speaking, either one of the movable plates of the sheet aligning member 41 moves in the sheet width direction, and abut the side edge of the discharged sheet P for moving the sheet P. The transportation position of the sheet P is therefore moved in the sheet width direction. Then, when the sheet P is completely moved, the paper feed unit 42 transports this sheet P to the first conveyance roller 25. When determining the transportation position of each sheet P, the motion amount and the motion direction in the sheet width direction are determined in accordance with the stacking position of the sheet P in the catch tray 51.

The paper feed unit 42 transports the sheet P stacked on the intermediate stacker 40 (or a plurality of sheets P stapled together) from this intermediate stacker 40 to the first discharging roller 25. The paper feed unit 42 is comprised of, for example, a pair of conveyer units located adjacent to each other in the sheet width direction. Each conveyer unit is comprised of a drive pulley, a driven pulley and a timing belt that is wound around these pulleys. The paper feed unit 42 transports the sheet P stacked on the intermediate stacker 40 to the first discharging roller 25 by driving the drive pulley to rotate the timing belt.

The staple unit 43 is comprised of a stapler which strikes staple legs through the sheets P and a clincher which bends the staple legs along the sheets P. This staple unit 43 performs a staple operation with a plurality of sheets P stacked on the intermediate stacker 40 in accordance with a predetermined position and a predetermined direction. For example, the staple unit 43 is used to perform side stitching by stapling the rear edges of a sheet stack.

The sheet stacker 50 is a device for stacking the sheet P discharged from the first discharging roller 25, and includes the catch tray 51 and an elevator mechanism 52. The catch tray 51 is used to successively stack the sheet P discharged from the first discharging roller 25, and designed to move up and down along the sheet stacking direction W1 by the elevator mechanism 52.

FIG. 2 is an explanatory view for schematically showing the configuration of the sheet stacker 50. Particularly, this explanatory view schematically shows the sheet stacker 50 with the catch tray 51 as viewed from above. The sheet stacker 50 of the present embodiment is further provided with a pair of regulating members 53 a and 53 b, and a holding unit which includes three holding members 54 a, 54 b and 54 c to perform a shift operation without misalignment. In this description, the shift operation is an operation of sorting and stacking the sheet P discharged from the first discharging roller 25 on a first stacking position or a second stacking position which is offset from the first stacking position in the sheet width direction W2 by a predetermined amount (shift amount). In the present embodiment, the shift operation is implemented as the transportation of the sheet P through the intermediate stacker 40. In other words, the sheet aligning member 41 enables the shift operation by determining in advance the transportation position of the sheet P in the sheet width direction according to the stacking position. Meanwhile, in this description, reference symbol “Pa” is used to specify the sheet P that is to be stacked in the first stacking position, and reference symbol “Pb” is used to specify the sheet P that is to be stacked in the second stacking position.

The pair of regulating members 53 a and 53 b are provided for aligning the sheet P discharged from the first discharging roller 25 in the sheet width direction W2. These regulating members 53 a and 53 b are arranged adjacent to each other in the sheet width direction W2. Also, each of the regulating members 53 a and 53 b is configured to move in the sheet width direction W2. It is thereby possible to adjust the distance between the pair of the regulating members 53 a and 53 b corresponding to the size of the sheet P in the sheet width direction W2, and adjust the positions of the regulating members 53 a and 53 b respectively corresponding to the stacking position of the sheet P.

Specifically speaking, when the sheet Pb is stacked in the second stacking position, one type of single-sided alignment is performed by fixing one regulating member 53 a and moving the other regulating member 53 b in the sheet width direction W2 in order that the sheet Pb abuts the one regulating member 53 a. Also, when the sheet Pa is stacked in the first stacking position, the other type of single-sided alignment is performed by fixing the other regulating member 53 b and moving the one regulating member 53 a in the sheet width direction W2 in order that the sheet Pa abuts the other regulating member 53 b.

Referring to FIG. 2 and FIGS. 3A and 3B, the single-sided alignment will be explained in a scenario that the first stacking position is switched to the second stacking position in which the sheet Pb is stacked. When stacking the sheet Pb in the second stacking position, the pair of regulating members 53 a and 53 b move from the positions as shown with broken lines in FIG. 2 to the positions as shown with solid lines corresponding to the second stacking position.

Then, while the regulating member 53 a as one of the pair is placed on the surface of the sheet Pa (the uppermost sheet Pa in the first stacking position) on the catch tray 51 as shown in FIG. 3A, the regulating member 53 b as the other of the pair is located in the position to push the side edge of the sheet Pb in the sheet width direction as shown in FIG. 3B. Incidentally, the pair of regulating members 53 a and 53 b are pivotally supported around a predetermined shaft, and their heads are held in predetermined lower limit positions by a stopper (not shown in the figure).

When the sheet Pb is discharged on the catch tray 51 as the first sheet to be stacked in the second stacking position, this sheet Pb is placed on the uppermost sheet Pa stacked in the first stacking position. Then, the other regulating member 53 b moves back and forward in the sheet width direction W2 to push the sheet Pb to abut the one regulating member 53 a. This motion of the other regulating member 53 b is performed each time the sheet P is discharged so that the single-sided alignment of the sheet Pb is performed during stacking the sheet Pb in the second stacking position.

Referring to FIG. 2 again, the three holding members 54 a through 54 c are arranged adjacent to each other in the sheet width direction W2. Each of the holding members 54 a through 54 c is constructed to move in the sheet width direction W2, and it is therefore possible to adjust the positions of the holding members 54 a through 54 c in accordance with the stacking position of the sheet P.

Specifically, of the three holding members 54 a through 54 c, the adjacent pair of the holding members 54 a and 54 b (or 54 b and 54 c) function as the holding members responsible for holding a tail edge as described below. The remaining holding member 54 c (or 54 a) functions as the holding member responsible for holding a shift position as described below. When the stacking position of the sheet P is switched, the functional assignment to the holding members 54 a through 54 c is switched in correspondence with the stacking position after switching.

As shown in FIGS. 4A and 4B, two of the holding members 54 a to 54 c assigned for holding a tail edge are driven to hold down the tail edge area of the sheet P from its upper side toward the sheet receiving surface (as a tail edge holding motion). In this description, the sheet receiving surface is the surface on which the discharged sheet P is placed. When no sheet has been placed on the catch tray 51, the catch tray 51 serves to provide the sheet receiving surface. When there is one or more sheet on the catch tray 51, the uppermost sheet P serves to provide the sheet receiving surface.

On the other hand, when switching the stacking position of the sheet P, one of the holding members 54 a to 54 c assigned for holding a shift position is driven to hold the side edge area of the uppermost sheet P, which is stacked in the stacking position before switching, in the side opposite the shift direction of the stacking position (as a shift position holding motion).

Referring to FIG. 2, it is assumed that the first stacking position is switched to the second stacking position in which the sheet Pb is stacked. In this scenario, of the rightmost and leftmost holding members 54 a and 54 c, the holding member 54 a which is located in the first stacking position side is used as the holding members for holding a shift position. Then, as shown with broken lines in FIG. 2, this holding member 54 a moves in the sheet width direction W2 by a predetermined amount from the position as shown with solid lines in FIG. 2 to the position as shown with broken lines. The holding member 54 a then holds the side edge area of the uppermost sheet Pa, which is stacked in the stacking position before switching, i.e., the offset area of the first and second stacking positions.

FIG. 5 is a block diagram for showing the control system of the image forming system in accordance with the present embodiment. The control system of this image forming system is comprised of an image formation control unit 100 and a finisher control unit 200. These control units 100 and 200 are designed to communicate with each other.

The image formation control unit 100 is responsible for integrally controlling the entire image forming system by controlling the image forming apparatus 1 as well as the finisher control unit 200. The image formation control unit 100 can be implemented with a microcomputer mainly including a CPU, a ROM, a RAM and an I/O interface.

The image formation control unit 100 acquires image data which is input from the image reading control unit 110 or the like. Also, the image formation control unit 100 controls these units of the image forming apparatus 1 on the basis of the input image data to form an image on the sheet P in correspondence with the input image data. Specifically speaking, the image formation control unit 100 controls an image forming unit 120, a paper feed unit 130 for feeding the sheet P from any one of the paper trays 17A, and a conveyance unit 140 respectively. In this case, the image forming unit 120 collectively represents the functional elements responsible for performing the image formation process, i.e., the photoreceptor unit 11, the charging unit 12, the image exposure unit 13, the charging/developing device 14, the transfer unit 15A, the separation unit 15B, the fixing unit 18 and so on.

The manipulation unit 150 outputs several parameters, which are selected by users, to the image formation control unit 100. The manipulation unit 150 is implemented, for example, with a touch panel through which a user can perform input operations with reference to information displayed on a screen. Through the manipulation unit 150, a user can enter printing conditions, for example, by selecting whether to perform a shift operation, setting or selecting the shift amount during the shift operation, selecting the type of the sheet P (for example, basis weight), selecting the density and reduce/enlarge ratio of images, setting whether to perform a post-printing process (staple operation), and so forth. Also, the image formation control unit 100 displays a variety of messages to the user through the manipulation unit 150 by controlling this manipulation unit 150.

The communication unit 160 serves as an input/output unit for establishing communication with the finisher 2 which is connected to own image forming system. The image formation control unit 100 can therefore communicate with the finisher control unit 200 of the finisher 2 through the communication unit 160.

The finisher control unit 200 is responsible for controlling the finisher 2. The finisher control unit 200 can be implemented with a microcomputer mainly including a CPU, a ROM, a RAM and an I/O interface.

The finisher control unit 200 controls a conveyance unit 210 for transporting the sheet P, a sheet alignment drive unit 220 for driving the sheet aligning member 41, a stapling drive unit 230 for driving the stapler and clincher of the staple unit 54, and the like. Also, the finisher control unit 200 controls an elevator drive unit 240 for driving up and down the catch tray 51 which is part of the elevator mechanism 52. The finisher control unit 200 thereby moves up and down the catch tray 51 in accordance with the stacking amount of the sheets P in order to maintain the height of the surface of the uppermost sheet P stacked on the catch tray 51. Furthermore, the finisher control unit 200 controls a regulating member drive unit 250 to move the pair of regulating members 53 a and 53 b in the sheet width direction W2.

The finisher control unit 200 also controls a holding member drive unit 260 to perform the tail edge holding motion and the shift position holding motion. In addition to this, the finisher control unit 200 is capable of controlling this holding member drive unit 260 to move the three holding members 54 a through 54 c in the sheet width direction W2 respectively. As described, the holding member drive unit 260 of the present embodiment serves as a holding drive unit for driving the holding members 54 a to 54 c respectively to perform the tail edge holding motion or the shift position holding motion, and as a lateral motion drive unit for moving the holding members 54 a through 54 c respectively in the sheet width direction.

In the case of the present embodiment, when the sheet P is sorted between the first stacking position and the second stacking position, the finisher control unit 200 determines the common area which is shared by the first stacking position and the second stacking position. The finisher control unit 200 then sets the positions of two of the holding members 54 a to 54 c for holding a tail edge in order that the tail edge holding motion is performed within the common area.

FIG. 6 is a view for explaining a scheme to determine the common area Rc and the positions of two of the holding members 54 a to 54 c for holding a tail edge. Specifically, the finisher control unit 200 determines the holding positions of two of the holding members 54 a to 54 c for holding a tail edge to positions F1 and F2 which are symmetrical about the center position Rcc of the common area Rc.

Particularly, in the present embodiment, the symmetrical positions F1 and F2 is determined to be center between the center position Rcc of the common area Rc and the edge positions of the common area Rc respectively. In this case, the symmetrical positions F1 and F2 are given by the following equations as distances Sa and Sb from the edges of the sheet P stacked on the second stacking position respectively.

Sa=((W+L)/2)+((W−L)/4)   Equation 1

Sb=((W+L)/2)−((W−L)/4)   Equation 2

In the above Equations, “W” is the extent of the sheet P in the sheet width direction W2, i.e., the width itself, and “L” is the amount of shift between the first stacking position and the second stacking position.

The finisher control unit 200 also receives a detection signal from a sheet detection unit 270. This sheet detection unit 270 is located, for example, in the vicinity of the first discharging roller 25, and capable of detecting the timing with which the sheet P is discharged from the first discharging roller 25.

A communication unit 280 is provided for establishing communication with the image forming apparatus 1 which is connected to the finisher 2. The finisher control unit 200 can thereby communication with the image formation control unit 100 of the image forming apparatus 1 through the communication unit 280.

FIG. 7 is a flowchart for showing the stacking operation of the present embodiment. Also, FIG. 8 is a timing chart which shows an example of the shift operation for switching the stacking position each time two sheets are stacked. The process shown in this flowchart is performed by the finisher control unit 200 in response to a print start command given by a user as a trigger (timing t0).

Incidentally, in the flowchart shown in FIG. 7, while focusing the description to the operations of the holding members 54 a to 54 c, other operations are omitted from the explanation. However, the finisher control unit 200 drives and moves a pair of the regulating members 53 a and 53 b in synchronization with the discharge of a sheet and switching of the stacking position, and moves the catch tray 51 downwards in accordance with the amount of the stacked sheets P.

First, in step 10(S10), the finisher control unit 200 determines whether or not the shift operation is selected as a printing condition which is requested by a user. If the determination is affirmative in step 10, i.e., if the shift operation is selected, the process proceeds to step 11(S11). Contrary to this, if the determination is negative in step 10, i.e., if the shift operation is not selected, the process proceeds to step 14(S14).

In step 11, the finisher control unit 200 determines whether or not there is a sheet P in the catch tray 51. If the determination is affirmative in step 11, i.e., if there is a sheet P in the catch tray 51, the process proceeds to step 17(S17). Contrary to this, if the determination is negative in step 11, i.e., if there is no sheet P in the catch tray 51, the process proceeds to step 12(S12).

In step 12, the finisher control unit 200 performs functional assignment of the holding members 54 a to 54 c. For example, when the first stacking position is used first as an initial setting, as shown in FIG. 2, the finisher control unit 200 assigns the adjacent pair of the three holding members 54 a through 54 c arranged in the first stacking position side, i.e., the holding members 54 a and 54 b as the holding members responsible for holding a tail edge.

In step 13(S13), the finisher control unit 200 determines the common area Rc which is shared by the first stacking position and the second stacking position on the basis of the size of the sheet P and the amount of shift. The finisher control unit 200 then moves the holding members 54 a and 54 b for holding a tail edge to the positions F1 and F2 as described above (timing t1: refer to FIG. 8).

In step 14(S14), the finisher control unit 200 determines whether or not the sheet P is discharged from the first discharging roller 25. If the determination is affirmative in step 14, i.e., if the sheet P is discharged from the first discharging roller 25, the process proceeds to step 15(S15). Contrary to this, if the determination is negative in step 14, i.e., if the sheet P is not discharged from the first discharging roller 25, the operation in step 14 is repeated.

In step 15, the finisher control unit 200 performs the tail edge holding motion with the sheet P discharged from the first discharging roller 25 (timing t2). The sheet P discharged from the first discharging roller 25 is thereby held down against the sheet receiving surface by two of the holding members 54 a to 54 c for holding a tail edge, after falling to the sheet receiving surface.

In step 16(S16), the finisher control unit 200 determines whether or not the sheet P just discharged is the last sheet of the print job. If the determination is affirmative in step 16, i.e., if the sheet P just discharged is the last sheet, the process ends. Contrary to this, if the determination is negative in step 16, i.e., if the sheet P just discharged is not the last sheet, the process is returned to step 10.

On the other hand, if there is a sheet P in the catch tray 51 in step 11 as described above, the finisher control unit 200 determines in step 17 whether to performs the shift operation, i.e., whether or not the sheet P to be discharged next is stacked in the stacking position to which the current stacking position is switched. If the determination is affirmative in step 17, i.e., if the shift operation is performed next, the process proceeds to step 18(S18). Contrary to this, if the determination is negative in step 17, i.e., if the shift operation is not performed next, the process proceeds to step 14 in which the tail edge holding motion is performed (for example, timing t3) on the condition that the sheet P is discharged from the first discharging roller 25.

In step 18, the finisher control unit 200 changes the functional assignment of the holding members 54 a to 54 c. For example, when the first stacking position is switched to the second stacking position, as shown in FIG. 2, the finisher control unit 200 assigns the adjacent pair of the three holding members 54 a through 54 c arranged in the second stacking position side, i.e., the holding members 54 b and 54 c as the holding members responsible for holding a tail edge. The finisher control unit 200 also assigns the remaining one of the three holding members 54 a through 54 c, i.e., the holding member 54 a arranged in the first stacking position side, which was selected before switching, as the holding member responsible for holding a shift position.

In step 19(S19), the finisher control unit 200 moves the three holding members 54 a through 54 c, and adjusts the positions of these holding members 54 a through 54 c in accordance with the assignment change (timing t4).

In step 20(S20), the finisher control unit 200 performs the shift position holding motion (timing t5) against the sheet P which is located in the uppermost position in the stacking position before switching, i.e., the sheet P which is currently located in the uppermost position. The sheet P stacked on the uppermost surface is therefore held down in the edge area by the holding member 54 a (or 54 c) for holding a shift position in the sheet stacking direction W1.

The process then proceeds to step 14 in which the tail edge holding motion is performed (for example, timing t6) on the condition that the sheet P is discharged from the first discharging roller 25. The above process is repeatedly performed until the last sheet is processed.

In accordance with the present embodiment as described above, when the sheet P is stacked after sorting between the first stacking position and the second stacking position, the finisher control unit 200 determines the common area Rc which is shared by the first stacking position and the second stacking position. The finisher control unit 200 then sets the positions of the pair of holding members (54 b and 54 c) for holding a tail edge, as shown in FIGS. 9A and 9B in order to perform the tail edge holding motion in the positions F1 and F2 respectively which are located within the common area Rc and symmetrical about the center position Rcc of the common area Rc.

FIG. 10 is an explanatory view for showing the situation where the tail edge holding motion is performed in a position outside the common area Rc. When the stacking position is switched by the shift operation, as shown in the same figure, even if a pair of the holding members is located on the uppermost sheet Pb, there is the possibility that one of the holding members holds in a position outside the stacking position before switching. In this case, there is concern that one of the holding members may displace the uppermost sheet Pb, and/or fold the sheet Pb to impair the sheet alignment.

On the other hand, the other holding member is located in the common area, and thereby the tail edge holding motion can be performed within the common area. However, in this case, the sheet P is held down only in a single position, and thereby when some external force is exerted on the sheet P by one holding member or some other cause, the sheet P may be rotated around the single position so that it is difficult to effectively inhibit the sheet misalignment.

On the other hand, from the view point of improving the productivity, the shorter the discharging interval of the sheet P, the faster it is required to perform the tail edge holding motion. In this case, two of the holding members 54 a to 54 c for holding a tail edge are driven by a high power, and thereby the tail edge area of the sheet P is held down by a relatively large force. Because of this, unless the sheet P is held down in an appropriate manner, some displacement of the sheet P may occur by the relatively large force of the holding members 54 a to 54 c.

In this regard, however, the pair of holding members for holding a tail edge according to the present embodiment are set in the positions located within the common area Rc and symmetrical about the center position Rcc of the common area Rc, so that the above shortcomings can be resolved. Namely, any holding member does not hold outside the stacking position before switching, and thereby it is possible to avoid the undesired situation that the sheet alignment level is lowered by displacing or folding of the uppermost sheet. Furthermore, the tail edge area of the sheet P is held down to keep a proper balance at two points, and thereby it is possible to effectively inhibit sheet misalignment.

Particularly, in accordance with the present embodiment, the symmetric positions F1 and F2 are determined at the middle positions between the center position Rcc of the common area Rc and the edge positions of the common area Rc respectively, and thereby it is possible to hold down the tail edge area of the sheet P with a proper balance. This effectively inhibits the sheet P from being displaced.

In addition to this, when switching the stacking position, the finisher control unit 200 of the present embodiment drives one of the holding members 54 a to 54 c assigned for holding a shift position to hold the edge area of the uppermost sheet P, which is stacked in the stacking position before switching, in the side opposite the shift direction of the stacking position (as the shift position holding motion).

FIG. 11 is an explanatory view for showing the sheet stacking state in which the holding member for holding a shift position is not used. As shown in the same figure, when switching the stacking position, the sheet Pa stacked in the stacking position before switching may be displaced by the sheet Pb discharged in the stacking position after switching. However, in accordance with the present embodiment, such displacement of the sheet P can be avoided in advance by performing the shift position holding motion.

Also, the finisher control unit 200 of the present embodiment assigns the holding members 54 a and 54 b (or 54 b and 54 c), which is an adjacent pair of the three holding members 54 a through 54 c, to the holding member for holding a tail edge, and assigns the remaining holding member 54 c (or 54 a) to the holding member for holding a shift position. Then, when switching the stacking position of the sheet P, the finisher control unit 200 changes the functional assignment of the three holding members 54 a to 54 c in correspondence with the stacking position after switching, and sets the positions of the holding members 54 a to 54 c in correspondence with the assignment change of the three holding members 54 a to 54 c and the stacking position after switching respectively.

This scheme dynamically changes assignment of the functions, i.e., the function to hold a tail edge and the function to hold a shift position, to the holding members 54 a to 54 c respectively in accordance with the stacking position, as shown in FIG. 12, rather than statically assigns the functions to all the holding members 54 a to 54 c respectively. By this scheme, even if there are a small number of the holding members 54 a to 54 c, the above holding operations can be effectively performed without compromising functional performance thereof, and thereby preventing the configuration of the system from being complicated and growing in size.

Furthermore, the finisher control unit 200 of the present embodiment sets the positions of the pair of regulating members 53 a and 53 b in correspondence with the stacking position of the sheet P.

The alignment of the sheets P in the stacking positions can be improved by providing the regulating members 53 a and 53 b controlled as has been discussed above. Particularly, the more the number of sheets stacked in the same stacking position, the more reliably the regulating members 53 a and 53 b are engaged with the discharged sheet P to attain a high alignment level. However, even with these regulating members 53 a and 53 b, since either of the regulating members 53 a and 53 b is in contact with or close to the sheet P stacked in the stacking position before switching, just after switching the stacking position, there may be a clearance between the sheet P stacked in the stacking position before switching and the regulating members 53 a and 53 b so that displacement of the sheet P may occur.

In this regard, according to the present embodiment, the above shortcomings can be appropriately inhibited by concurrently performing the tail edge holding motion. On the other hand, while the regulating members 53 a and 53 b move in synchronization with switching of the stacking position and thereby is not effective to control the sheet P before switching, the shift position holding motion is concurrently performed to inhibit sheet misalignment in a more effective manner.

Meanwhile, in accordance with the present embodiment, the finisher control unit 200 may set a holding pressure, with which the shift position holding motion is performed by the holding members 54 a to 54 c, to be smaller than the holding pressure with which the tail edge holding motion is performed by the holding members 54 a to 54 c.

This scheme makes it possible to gently hold down the sheet P, and thereby inhibit the shortcomings that, when the shift position holding motion is performed, the sheet P is displaced by this operation of holding down the sheet P with the holding members 54 a to 54 c.

Meanwhile, in the case of the embodiment as described above, the shift operation is performed by offsetting the transportation position of the sheet P. However, the present invention is not limited to this. For example, the catch tray may be moved in the sheet width direction in synchronization with switching of the stacking position. Alternatively, in synchronization with switching of the stacking position, the sheet P may be moved in the sheet width direction by the pair of the regulating members 53 a and 53 b.

The foregoing description has been presented on the basis of the embodiments. However, it is not intended to limit the present invention to the precise form described, and obviously many modifications and variations are possible within the scope of the invention. While the sheet stacker is controlled by the control unit of the finisher in accordance with the embodiment as described above, the sheet stacker itself may be provided with own control unit, or may be controlled by the control unit of the image forming apparatus. Furthermore, while the sheet stacker is implemented in the finisher in accordance with the embodiment as described above, it is possible to implement the sheet stacker in the image forming apparatus or the like. 

1. A sheet stacker comprising: a catch tray configured to stack a sheet discharged from a discharging unit; a holding unit including a first holding member and a second holding member which are arranged adjacent to each other in the sheet width direction that is perpendicular to the discharge direction of the sheet; a lateral motion drive unit configured to move said first holding member and said second holding member in the sheet width direction respectively; a holding drive unit configured to drive said first holding member and said second holding member to hold down the tail edge area of the sheet from the upper side of the sheet toward a sheet receiving surface; and a control unit configured to control said holding drive unit to perform a tail edge holding motion in synchronization with the discharge of the sheet, and control said lateral motion drive unit, wherein, when the sheet is stacked by sorting between a first stacking position and a second stacking position which is offset from said first stacking position in the sheet width direction, said control unit determines a common area which is shared by said first stacking position and said second stacking position, and sets the positions of said first holding member and second holding member in order that said tail edge holding motion is performed in symmetric positions which are located within said common area and symmetrical about the center position of said common area.
 2. The sheet stacker of claim 1 wherein said holding unit comprises three holding members functioning as said first holding member, said second holding member and a third holding member which is arranged in said sheet width direction with said first holding member and said second holding member, and wherein, when said stacking position is switched, said control unit performs a shift position holding motion by holding the side edge area of an uppermost sheet stacked in the stacking position before switching with said third holding member, in the side opposite the shift direction of said stacking position.
 3. The sheet stacker of claim 2 wherein said control unit assigns, of said three holding members constituting said holding unit, a pair of adjacent holding members to said first holding member and said second holding member for performing said tail edge holding motion and the remaining holding member to said third holding member for performing said shift position holding motion.
 4. The sheet stacker of claim 3 wherein, when said stacking position is switched, said control unit changes assignment of said three holding members constituting said holding unit in correspondence with the stacking position after switching, and wherein said control unit sets the positions of said three holding members in accordance with said assignment thereof as changed and the stacking position after switching.
 5. The sheet stacker of claim 2 wherein said control unit sets a holding pressure, with which said shift position holding motion is performed by said holding members, to be smaller than a holding pressure with which said tail edge holding motion is performed by said holding members.
 6. The sheet stacker of claim 1 further comprising: a pair of regulating members configured to align a sheet discharged from said discharging unit in the sheet width direction; and a regulating member drive unit configured to move said regulating members in the sheet width direction respectively, wherein said control unit controls said regulating member drive unit to set the positions of said pair of regulating members in correspondence with a stacking position of the sheet.
 7. A finisher comprising: a sheet stacker; a sheet receiving unit configured to receive a sheet on which an image is formed from an image forming apparatus; a post-printing unit configured to perform a predetermined post-printing process with the sheet received through said sheet receiving unit; and a discharging unit configured to discharge the sheet from said post-printing unit or from said sheet receiving unit to said sheet stacker, wherein said sheet stacker comprises: a catch tray configured to receive the sheet discharged from said discharging unit for stacking the sheet thereon; a holding unit including a first holding member and a second holding member which are arranged adjacent to each other in the sheet width direction that is perpendicular to the discharge direction of the sheet; a lateral motion drive unit configured to move said first holding member and said second holding member in the sheet width direction respectively; a holding drive unit configured to drive said first holding member and said second holding member to hold down the tail edge area of the sheet from the upper side of the sheet toward a sheet receiving surface; and a control unit configured to control said holding drive unit to perform a tail edge holding motion in synchronization with the discharge of the sheet, and control said lateral motion drive unit, wherein, when the sheet is stacked by sorting between a first stacking position and a second stacking position which is offset from said first stacking position in the sheet width direction, said control unit determines a common area which is shared by said first stacking position and said second stacking position, and sets the positions of said first holding member and second holding member in order that said tail edge holding motion is performed in symmetric positions which are located within said common area and symmetrical about the center position of said common area. 